Moisture swing sorption is a novel sorbent cycle technology that uses water rather than heat or pressure to modify the loading of sorbent. Previously, moisture swing behavior has previously been shown in certain anionic exchange resins that have quaternary ammonium ions as a fixed positive charge, and can use hydroxides, carbonate or bicarbonate ions as a negative charge. For a material to have a moisture swing, two characteristics are important. First, the material needs to have the capacity to absorb CO2 from a gas, and second, the affinity to CO2 should be reduced by the presence of water vapor (thereby allowing selective and controllable CO2 capture and release). Many materials have the opposite of this type of moisture swing, whereby the affinity to CO2 is increased by reducing the water content.
Metal organic frameworks (MOF) materials comprising metal ions or metal ion clusters coordinated to polydentate organic ligands have some of the highest pore volume, surface area, and lowest densities known. Consequently, MOFS have been extensively studied for a myriad of applications such as sensing, catalysis, separations, and gas storage. Of the tens of thousands of known MOF materials, particularly stable transition metal-based MOFS such as Zirconium based MOFs (ZrMOF), including UiO-66 (also called Zr-BDC) are becoming more popular due in part to their superior stability relative to MOFs based on other metals. Other exceptionally stable MOF materials of interest include chromium (III) terephthalate (MIL-101), which possesses notable stability and resistance to leaching of metal ions in organic and aqueous solvents and at high temperatures. These materials could offer the potential for use in gas storage and separation under real-world use conditions that include a positive moisture swing mechanism.